Why there's a formula at all
Federal law requires every state to set child support guidelines, review them at least every four years, and treat the guideline amount as presumed correct (42 U.S.C. 667). A court can order a different amount only with a written finding that the guideline amount would be unjust or inappropriate in that case (45 CFR 302.56(f)-(g)).
Federal rules also require each state's guideline to look at the paying parent's income and ability to pay, include a low-income adjustment such as a self-support reserve, address the children's health care, and not treat incarceration as voluntary unemployment (45 CFR 302.56(c)). Within those rules, each state picks its own model and numbers.
The three main models
Income shares (40 states and DC)
The idea: children should get the share of income they would have received if the parents lived together. The state publishes a schedule of what parents with a given combined income are presumed to spend on their children. The court adds both parents' incomes, looks up the amount for that income and number of children, and splits it by each parent's share of the combined income. The parent who has the children most of the time is presumed to spend their share directly; the other parent pays theirs.
Example from Florida's schedule: combined net income $5,000 a month, one child: $1,000. A parent who earns $3,000 of the $5,000 owes 60%, or $600.
Many income shares states adjust for shared parenting time once a parent passes a threshold of overnights: 73 a year (20%) in Florida, 146 in Illinois. Indiana gives a parenting time credit starting at 52 overnights.
Percentage of income (six states)
The guideline is a percentage of the paying parent's income only, set by the number of children. Alaska, Mississippi, Nevada, North Dakota, Texas and Wisconsin use this model. In Texas, for example, it's 20% of the paying parent's net resources for one child and 25% for two, up to a cap on net resources.
Example from Texas: a parent earning $5,000 a month in wages has $4,199.17 of net resources under the Attorney General's 2026 tax chart; 20% for one child is $839.83.
Melson formula (three states)
A variation on income shares that first sets aside a self-support reserve for each parent, then covers the children's basic needs, and then shares a percentage of any remaining income (a "standard of living adjustment") so children benefit when a parent earns more. Delaware names the Melson formula in its guidelines; Hawaii and Montana use the same structure.
California's own formula
California uses CS = K[HN − (H%)(TN)], written into Family Code section 4055. It combines both parents' net incomes with the higher earner's share of time with the children in one formula, so time-share affects every result rather than only past a threshold.
The Judicial Council's example: net incomes of $6,000 and $4,000, the higher earner with the children 20% of the time: K = 0.30, support = 0.30 × ($6,000 − 20% × $10,000) = $1,200 a month.
Which model each state uses
Each row links the official text we read for that state: usually its guideline statute or court rule, and for a few states (such as Montana, Ohio and Indiana) a state agency or legislative document that describes the guideline. Some states describe an income shares design without using the label; we list what the text describes. Mississippi changed how support is figured for equal-time joint custody from July 1, 2026 (HB 1662, chapter 467).
What goes into the calculation
- Income. Usually all income from all sources. Some states start from gross income, others from net income after taxes, and the definitions differ.
- Number of children the order is for.
- Parenting time, in most states, measured in overnights a year.
- Add-ons: health insurance for the children, and often child care for work and uninsured medical costs, split between the parents.
- Other children a parent supports, which many states credit.
Courts can also count income a parent could earn but doesn't (imputed income) and can order a different amount for reasons the state's law lists. A calculator can't decide those questions; it shows what the guideline formula gives.
How is child support calculated?
Each state uses the formula in its own guideline law. In most states (40 and DC), the court adds both parents' incomes, looks up a schedule amount for that combined income and number of children, and splits it by each parent's share of income. Six states take a percentage of the paying parent's income, three use the Melson formula, and California uses its own formula.
Which states use the income shares model?
40 states and the District of Columbia, by our count from each state's guideline text. The others are Alaska, Mississippi, Nevada, North Dakota, Texas and Wisconsin (percentage of income), Delaware, Hawaii and Montana (Melson formula), and California (its own K-factor formula). The table on this page links each state's rule.
Can a judge order more or less than the guideline?
Yes. Federal law makes the guideline amount a rebuttable presumption: a court can order a different amount with a written finding, under criteria the state sets, that the guideline amount would be unjust or inappropriate in that case (42 U.S.C. 667(b)(2)).
Does parenting time change child support?
In most states, yes, once a parent has the children for a set number of overnights: for example 73 a year in Florida and 146 in Illinois, while Indiana's credit starts at 52. California counts time-share in every case. Texas's percentage guideline doesn't change with time, though a court may consider it as a reason to vary from the guideline.
Updated November 8, 2026. Sources: 42 U.S.C. 667; 45 CFR 302.56; each state's guideline statute, court rule or agency guideline as linked in the table (retrieved November 8, 2026); Tex. Fam. Code ch. 154; Cal. Fam. Code §4055; Fla. Stat. §61.30.